Hot water is pumped by a suction pump and introduced into a carbon dioxide (CO2) gas dissolver through solution flow rate adjusting means and then poured into a bath. The CO2 is introduced into the CO2 gas dissolver through gas flow rate adjusting means, and the quantity of bubbles in an artificial carbonated spring in a take-out pipe is measured. The solution and gas flow rate adjusting means are controlled via a control device using a relationship expression between a preliminarily set quantity of bubbles and carbon dioxide concentration to obtain a desired concentration of carbon dioxide gas in the carbonated spring. Since the carbon dioxide gas flow control means is provided between the carbon dioxide gas dissolver and a carbon dioxide gas supply source, a high concentration carbonated spring can always be manufactured, even if the pressure of supplied carbon dioxide gas changes or membrane permeation changes.